Spark ignition, internal combustion engines are in universally widespread use throughout the world. Such engines are used in automobiles, lawnmowers, lift vehicles, etc. In a typical spark ignition, internal combustion engine, fuel is mixed with air and is drawn into a combustion chamber. The air-fuel mixture is compressed and then an electric spark is provided to very rapidly combust the air-fuel mixture, thereby providing useful power as the rapidly-expanding, combusting gases push down on the top of the piston.
These engines require three basic components for proper operation. Firstly, an appropriate mixture of air and fuel is required. Secondly, the air-fuel mixture needs to be compressed to a certain minimum pre-combustion pressure. Thirdly, a sufficient electric spark (or other ignition initiator) is needed to ignite the compressed air-fuel mixture. The absence of any one of these three elements typically renders an engine inoperable (or results in an engine that will not operate properly). To stop the engine when desired, it is standard that the electric spark is taken away. This is accomplished by, for example, turning off the engine by switching a keyswitch from the xe2x80x9conxe2x80x9d position to the xe2x80x9coffxe2x80x9d position. This has the effect of de-energizing the electric ignition system, thereby instantaneously denying the engine an electric spark, and shutting down the engine.
However, this method of shutting off an engine by simply turning off the electric spark can sometimes result in xe2x80x9cbackfiringxe2x80x9d of the engine. This is so because even though the electric spark has been taken away, the engine continues to turn over due to its own inertia, and the continued operation of the engine (albeit without the electric spark) continues to draw fuel and air into the engine. If the engine has internal hot spots, such hot spots can ignite the air-fuel mixture despite the absence of any electric spark. Such hot spots may occur, for example, on exhaust valves, on the crown of a piston, or on the cylinder head wall. Oftentimes, such hot spots are the result of the build-up of carbon deposits, which carbon deposits, once heated, do not dissipate that heat very quickly. As the engine begins to slow toward a stop, a charge of air and fuel within one or more of the combustion chambers can auto-ignite due to the hot spots, as mentioned above. If this combustion from auto-ignition occurs while either an intake valve or exhaust valve is open, xe2x80x9cbackfiringxe2x80x9d can be propagated through the open valve and into the intake or exhaust system. This backfiring is quite loud and unnerving to the operator of the vehicle and to others in the vicinity. This loud, unexpected noise is particularly bothersome when it occurs inside a building, rather than outside, such as when cars, trucks, or lift vehicles are operated within a factory. Such backfiring can also damage the engine.
To address this problem, it is known, in addition to denying the engine any further spark, to shut off the flow of fuel as early as possible after the keyswitch is moved to the xe2x80x9coffxe2x80x9d position. An example of such an arrangement is shown in U.S. Pat. No. 5,357,935 of Oxley et aL. The ""935 patent describes an engine xe2x80x9cshutdown circuitxe2x80x9d which includes a solenoid-operated three-way valve connected to an electrical system such that when the key switch is switched off, delivery of fuel is interrupted as quickly as possible.
However, even if a system were devised that could operate to interrupt fuel delivery immediately upon movement of the key switch from the xe2x80x9conxe2x80x9d position to the xe2x80x9coffxe2x80x9d position, such a system still cannot guarantee that the supply of fuel and air is instantaneously denied to the engine. This is so because there is a certain amount of air and fuel already in the fuel system downstream of where it can be cut off, and as the engine is shut down, it takes a finite amount of time for this air and fuel to work its way into the engine, there to be consumed. Thus, even though one can instantaneously shut off the supply of electric spark to the engine, it takes a finite amount of time before typical fuel systems can shut off the supply of fuel to the engine. Thus, for a short time, the engine is operated with no spark, but yet with air and fuel still supplied to the engine, thereby creating conditions that are still ripe for backfiring.
Accordingly, it can be seen that a need yet remains for a technique and apparatus for minimizing the likelihood of backfiring within spark ignition, internal combustion engines. It is to the provision of such a method and apparatus that the present invention is primarily directed.
Briefly described, in one aspect the present invention comprises a method for operating a spark ignition, internal combustion engine to minimize the likelihood of backfiring as the engine is turned off. The engine is of the type having an electrical ignition system for providing an ignition spark to the engine and further having a fuel system for admitting fuel into the engine. The fuel system is of the type having a cut-off device for stopping the flow of fuel into the engine in response to an engine shut-off control signal, such as when the on-off switch is moved from the xe2x80x9conxe2x80x9d position to the xe2x80x9coffxe2x80x9d position.
The method of the invention comprises the step of maintaining the ignition spark to the engine despite the presence of an engine shut-off control signal and continuing to do so for a predetermined length of time after shut-off. The length of time is selected to allow the fuel cut-off device to operate to stop the flow of fuel into the engine. In this way, for as long as the engine is turning over, the engine is provided with an electric spark to prevent or avoid backfiring.
Preferably, the length of time for maintaining the ignition spark to the engine is on the order of several seconds. Preferably, the length of time is greater than one second or so. More preferably, the pre-determined length of time is more than one second and less than about ten seconds. Most preferably, the length of time for maintaining the ignition spark to the engine after the generation of an engine shut-off control signal is between about three and six seconds.
In another aspect, the present invention comprises an engine control system for use with a spark ignition, internal combustion engine to minimize backfiring of the engine as the engine is turned off. The engine is of the type having an electric ignition system for providing ignition spark to the engine and a fuel system. The engine control system preferably comprises a switch which is selectively operable between an xe2x80x9conxe2x80x9d position for running the engine and an xe2x80x9coffxe2x80x9d position for turning off the engine. The engine control system preferably further includes a fuel shut-off device operable for interrupting delivery of fuel to the engine from the fuel system. The engine control system preferably further includes a control unit which is responsive to movement of the switch from the xe2x80x9conxe2x80x9d position to the xe2x80x9coffxe2x80x9d position for maintaining operation of the electric ignition system for a length of time after the switch has been moved to the xe2x80x9coffxe2x80x9d position. The control unit preferably also is operable for selectively operating the fuel shut-off device for interrupting delivery of fuel from the fuel system to the engine upon the switch being moved from its xe2x80x9conxe2x80x9d position to its xe2x80x9coffxe2x80x9d position.
In a further preferred embodiment, the control unit is operative to maintain operation of the electric ignition system for several seconds after the switch is moved to the off position. Preferably, the control unit is operative to maintain operation of the electric ignition system for more than 1 second and less than about 10 seconds. Most preferably, the control unit is operative to maintain operation of the electric ignition system for between about 3 seconds and 6 seconds. Preferably, the control unit comprises a delay-on-break timer.
Preferably, the fuel shut-off device is actuated by an engine vacuum source and the invention further includes a 3-way vacuum switch controlled by the control unit to effect control of the fuel shut-off device.
In an example embodiment, the engine is preferably of the type powered by liquid propane and the fuel shut-off device is placed in-line between a liquid propane tank containing liquid propane and a mixer in which air is mixed with propane for delivery to the engine. More preferably, the fuel shut-off device is placed in-line between the tank and a converter in which liquid propane is expanded to a gaseous state for delivery to the mixer.
Preferably, the fuel shut-off device includes a valve and a diaphragm for opening and closing the valve, and wherein with atmospheric pressure applied to one side of the diaphragm the valve is forced open and wherein with reduced pressure from the engine applied to the one side of the diaphragm the valve is forced closed. More preferably, the invention also includes an electrically controlled 3-way vacuum switch, which is electrically controlled by the control unit, and which is in fluid communication with atmosphere, engine vacuum, and the fuel shut-off device.
In yet another aspect, the present invention is an internal combustion engine comprising an electric ignition system; a fuel system; a switch operable between xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d positions; a fuel shut-off device operable to interrupt delivery of fuel from said fuel system; and a control unit for maintaining operation of said electric ignition system for a length of time after said switch is moved from the xe2x80x9conxe2x80x9d position to the xe2x80x9coffxe2x80x9d position and for actuating said fuel shut-off device to interrupt delivery of fuel from said fuel system upon said switch being moved from the xe2x80x9conxe2x80x9d position to the xe2x80x9coffxe2x80x9d position.
In further preferred form, the control unit is a delay-on-break timer. In still further preferred form, the fuel shut-off device is a pressure-actuated valve, and the engine further includes a vacuum switch in fluid communication with the pressure-actuated valve.
These and other objects, features and advantages of the present invention are described herein with reference to various preferred and example embodiments.